KR20080006877A - Method for manufacturing high strength and high ductility magnesium sheets using differential speed rolling at low temperatures - Google Patents

Abstract

A method for manufacturing high strength and ductility magnesium sheets using differential speed rolling at low temperatures is provided to improve efficiency of rolling by precluding damage of the magnesium sheets at ambient temperature. A method for manufacturing high strength and ductility magnesium sheets using differential speed rolling at low temperatures comprises the steps of: preparing pre-heated magnesium sheets at room temperature or below temperature of 100°C; executing differential speed rolling for the magnesium sheets; annealing the rolled magnesium sheets. The ratio of rotation speed of two rolled rolls opposed in differential speed rolling is 1.1 : 1 to 10 : 1.

Description

Manufacturing method of high strength and high ductility magnesium plate by low temperature double speed rolling {METHOD FOR MANUFACTURING HIGH STRENGTH AND HIGH DUCTILITY MAGNESIUM SHEETS USING DIFFERENTIAL SPEED ROLLING AT LOW TEMPERATURES}

1 is a schematic diagram showing the concept of double-speed rolling.

The present invention relates to a low temperature double-speed rolling method for significantly increasing the strength and ductility of a magnesium alloy, and more particularly to a plate of a magnesium alloy, which is known to be impossible to process at low temperatures due to the brittle nature, of 100 ° C. or less. By rolling at low temperature, the present invention relates to a method of obtaining a magnesium sheet having high strength and high ductility of up to 300 MPa in tensile strength and a maximum elongation at room temperature of 40%.

Magnesium is a plastic-level lightweight material that has electromagnetic wave shielding effects and is easily recycled compared to conventional plastic materials, and is therefore highly valuable as an environmentally friendly material.

However, pure magnesium or magnesium to which a small amount of solute element is added has a ductile structure of limited hexagonal lattice (HCP) at room temperature. Therefore, the room temperature processing of magnesium material is difficult due to the low ductility due to the crystal structure of magnesium.

Therefore, the die casting method, which is one of the casting methods, has been conventionally used to produce magnesium material in a desired shape. Die casting is also called die casting. It is a precision casting method in which molten metal is injected into a steel mold precisely machined to perfectly match the required casting shape to obtain the same casting as the die. It is called.

However, when magnesium is manufactured by the die casting method, a material having a thickness of 0.6 mm or less is difficult to produce, and the surface is not beautiful, and the strength and ductility are low, so that the shock absorbing ability is low. Therefore, there is a need for a method of molding a product through plastic molding of magnesium plate material.

Conventionally, in order to increase the ductility of magnesium, when the magnesium is processed at high temperature by using extrusion or rolling heat treatment, etc., the grain size of magnesium is fine, the strength and ductility are increased.

However, the effect of grain refining alone is not sufficient for the ductility of magnesium material when processing with high degree of deformation, and the plate material obtained through rolling and extrusion has a large material anisotropy, so that the difference in strength and elongation in each direction is large. It was difficult to replace the aluminum parts that are being molded at the level.

In order to solve this problem, the inventors of the present invention have proposed a method of manufacturing a magnesium plate material having low material anisotropy and excellent ductility in Korean Patent Application No. 10-2006-0011315. The manufacturing method of the magnesium plate is a method of increasing the ductility and isotropy of the magnesium material by rolling the magnesium material, the manufacturing method of the magnesium plate to perform a double-rolling on the magnesium material, the temperature of the plate during the double-speed rolling It is characterized by maintaining in the range. In this case, the continuous rolling is different from the upper and lower rolling roll diameter, or when the upper and lower rolls of the same diameter is applied to the upper and lower rolls, as shown in FIG. By imparting extreme shear stress to materials to be rolled at different speeds, it refers to a method of obtaining new textures and ultrafine grains that cannot be obtained by the conventional rolling method.

In this case, as the special aggregate structure and microcrystalline structure are developed inside magnesium, the processability at room temperature and high temperature is improved, so that it is not only applicable to fields such as structural materials or exterior materials, which are common uses, but also difficult to process due to its complicated shape. It has the advantage that the part can be effectively molded at room temperature or lower than the existing molding temperature.

However, even in the case of continuous rolling, it is necessary to maintain the material at a temperature of 100 ° C. or higher in order to increase the ductility of the brittle magnesium material at the time of rolling. Heating of the material or heating of the roll may shorten the life of the roll and may increase the production cost.

The present invention is to solve the above problems, even when rolling the magnesium material at room temperature can be smoothly rolled without breaking the magnesium material, the resulting magnesium material to have a high ductility and strength Its purpose is to provide a new method of double speed rolling.

Method of the present invention for achieving the above object, the step of preparing a magnesium plate preheated to room temperature or less than 100 ℃; Performing a continuous rolling on the magnesium sheet; And annealing the accelerated rolled magnesium sheet.

At this time, the rotational speed ratio of the two rolling rolls facing at the time of the continuous rolling is preferably between 1.1: 1 to 10: 1.

In addition, the rolling reduction rate per pass during the rolling speed is preferably 5 to 30%.

In addition, when the preheating temperature of the magnesium plate is 60 ~ 100 ℃ it is preferable that the reduction rate per pass during the continuous rolling is 50% or less.

In addition, the temperature of the annealing treatment step is more preferably 100 ~ 450 ℃.

In addition, the treatment time of the annealing treatment is effective for 2 minutes to 4 hours.

Hereinafter, the present invention will be described in detail.

The magnesium material targeted in the present invention is 70wt% of magnesium It means magnesium alloy or pure magnesium which is above. If the magnesium material is a magnesium alloy, there is no particular limitation on the remaining additive elements. For example, the remaining materials other than magnesium may include zinc, aluminum, manganese, or rare earth elements depending on the purpose. In addition, pure magnesium or magnesium alloy may contain some inevitable impurities. Examples of the magnesium alloy include AZ91, AZ31, ZK60, Mg-RE (rare earth), and the like.

The inventors of the present invention have studied the method of increasing the ductility by a simple method without additional heating equipment or heating process, as much as possible, and in the case of giving a rolling reduction in a limited range even at room temperature, the maximum temperature without heating or heating up to 100 ° C. It was confirmed that it could be rolled up to a reduction ratio per pass of 30%.

In other words, when the magnesium sheet is continuously rolled while giving a reduction amount of 5 to 30% per pass, rolling can be performed without heating the plate at all, and a rolled product having greatly improved strength can be obtained.

The reason for this is as follows. That is, in the case of constant-rolling or general extrusion of magnesium plate, the normal direction (ND) // (0002) fiber assembly structure in which the basal plane is parallel to the plate surface is developed very strongly. The moldability is lowered. In the case of the application of double-speed rolling, the development of such aggregates is suppressed compared to the constant-speed rolling, while the aggregates are developed in which the base surface rotates about 5 to 15 ° from the vertical direction to the rolling direction. This is due to the characteristics of shear deformation due to double-speed rolling. In this case, the strength of the fiber assembly structure originally formed during constant velocity rolling is lowered, and an aggregate structure is formed in which the base surface is rotated from the rolling direction, thereby twinning during rolling. ) And the slip is likely to occur, and at the same time, an effective grain refining effect due to shear deformation can be obtained, thereby greatly improving the low temperature rolling property of the material. In addition, the low temperature rolling property of the material is improved by the heat generated in the material itself due to the firing work due to the high shear processing amount acting at the time of the continuous rolling.

In addition, in the case of the continuous rolling within the limited rolling reduction range, although the rolling reduction and the cumulative rolling reduction per pass are lower than the continuous rolling at high temperature, the degree of ductility improvement due to the strength increase or further heat treatment is higher than the high rolling speed. It is as good as rolling.

In other words, due to the characteristics of the low-temperature biaxial rolling, the grain refinement effect after recrystallization is high due to the internal texture development effect and the increase of dislocation density due to the suppression of recovery during rolling.

Therefore, the low temperature double-speed rolling is an advantageous method for improving the strength, ductility, and grain refinement of a plate whose processing amount is limited. In other words, in high speed double rolling, the desired rolling effect may be increased, whereas in the low speed double rolling, when the rolling reduction is not large, that is, the thickness of the material to be processed initially is thin, or, conversely, the final product is thick. Even if it is not large, there is an advantage that a sufficient effect can be obtained.

In addition, the effect of low temperature continuous rolling on such magnesium plate is more pronounced than that of low temperature constant velocity rolling, which means that the strength of the base surface texture developed during rolling is lower than that of constant velocity rolling, thereby promoting twin development. This is because it is advantageous to secure the minimum ductility required for rolling.

In addition, it is possible to increase the relative amount of processing compared to the constant velocity rolling for the above reason, since the resulting work also becomes larger than the constant velocity rolling, so that the heat supply effect by the firing date is also better than the constant velocity rolling. Therefore, in the case of performing the continuous rolling, it is possible to add up to about 2 to 10 times the rolling reduction per pass than the constant velocity rolling, so that the effect of developing the texture and grain refinement in a direction favorable for processing increases.

And, in addition to the ductility increase effect, if the rolling speed ratio during two-speed rolling within a certain range as described above, the anisotropy of the plate material is significantly reduced, and thus the same strength and ductility can be obtained regardless of the direction in the plate surface . This is a peculiar phenomenon only in two-speed rolling in the proper speed range, which is difficult to expect when the rolling speeds of the upper and lower rolls are the same for the magnesium alloy of the general hcp structure.

In order to obtain such an advantageous effect, it is necessary to maintain the rolling rate ratio between the upper and lower rolling rolls applying the rolling force to the magnesium sheet between 1.1: 1 and 10: 1. If the ratio of the rolling speed is lower than the ratio, the aggregate structure to be obtained in the present invention is not sufficiently formed and the grain refining effect is small, so that the ductility increase and the material isotropic improvement effect are insufficient, and sufficient firing work amount is not secured. Therefore, there is a risk of breaking the material during low temperature continuous rolling. In addition, when the rolling speed ratio is higher than 10: 1, the increase in ductility can be expected to some extent, but it is difficult to expect the effect of reducing the anisotropy anymore, and the difference in the amount of upper and lower processing of the plate is severe and the processed sheet may be damaged. And it is not preferable because the bending of the plate tends to be very severe. Therefore, it is preferable to maintain the rolling rate ratio of the upper and lower rolls at 1.1: 1 to 10: 1 at the time of continuous rolling.

In addition, it is necessary to maintain the rolling reduction rate per pass during the low temperature continuous rolling in accordance with the present invention between 5 and 30%. If the reduction ratio is less than 5%, the problem of grain refinement, the development of aggregate structure and dislocation density of the shape intended in the present invention is not sufficient, and the shear deformation is not uniformly distributed in the thickness direction, so that the mechanical properties of the sheet are not uniform. May occur, and the plate may be damaged if it exceeds 30%.

As mentioned above, according to the manufacturing method of the magnesium plate material of this invention, it is not necessary to preheat a material. In very limited cases, however, double-rolling may be carried out after preheating of the material at temperatures below 100 ° C. Also in this case, since the said preheating temperature should just be 100 degrees C or less, it differs from normal high temperature continuous rolling. The limited case referred to here is for increasing the reduction amount per pass, for example, when preheating only the sheet at 60 to 100 ° C., the maximum reduction ratio may be increased to 50%. When preheating the material, the preheating time is not particularly limited, but it is only necessary to keep the temperature of the material sufficiently uniform.

The yield strength of the magnesium sheet through the bi-speed rolling process can be increased to 260 ~ 280MPa, tensile strength up to about 280 ~ 330MPa. However, in order to manufacture a plate in a desired form through a process such as pressing directly, an additional heat treatment is necessary because the ductility of the plate is still insufficient, and the additional heat treatment is preferably annealing. In other words, annealing and recrystallization effect may be obtained when the fast-rolled magnesium sheet is maintained at a constant temperature for a predetermined time. The annealing and recrystallization improves the work hardening ability of the material, and the accumulated stress state inside the rigid plastic The first fiber assembly that had been relaxed and formed inside the magnesium material before rolling could be further dismantled, resulting in further increased ductility of magnesium.

The temperature of the annealing step to obtain the above effects is preferably 100 ~ 450 ℃. If the temperature is less than 100 ℃, it is difficult to obtain the effect of dislocation density reduction and stress relaxation, when the temperature exceeds 450 ℃ is due to the loss of ductility increase effect due to the texture and recrystallization formed by the rapid rolling. The annealing time is set to the upper and lower limits of 2 minutes or more and 4 hours or less. When the annealing time is set to 2 minutes or less, the annealing time is too short to obtain the annealing effect described above. This is because there is no ductility increase effect, but energy loss occurs.

(Example)

In order to confirm the difference between low temperature constant speed rolling and low temperature double speed rolling, AZ31 magnesium alloy plate (Al: 3wt%, Zn: 1wt%, Mg: 96wt%) having a thickness of 0.8mm was subjected to constant speed rolling and double speed rolling. The speed ratio of the upper roll and the lower roll during double speed rolling was set to 3: 1. During the rolling, the material was not preheated or heated. As a result of observing the reduction rate per pass without breaking when increasing the roll load during the constant velocity rolling, the fracture occurred in the plate at the reduction rate of 5% or more per pass in constant velocity rolling, but 22% in the case of continuous rolling. No breakage occurred even in the reduction ratio. Therefore, it could be confirmed that rolling by a relatively large reduction ratio can be achieved by the speed rolling.

Tensile experiments were performed after annealing treatment at 200 ° C. for 0 minutes, 2 minutes, and 30 minutes, respectively, on the magnesium plate rolled at the reduction ratio of 22%, and the results are shown in Table 1 below.

Annealing time Yield strength (MPa) Tensile Strength (MPa) Elongation at break (%) 0 min 280-310 330-340 10 2 minutes 260-280 280-310 20 30 minutes 200-220 280-300 40

As can be seen in Table 1, the plate treated for 2 minutes at 200 ℃ yield strength: 200 ~ 280MPa, tensile strength 280 ~ 310MPa of high strength, the maximum elongation is 20%. The maximum elongation is a relatively high value of elongation, considering that the strength of the magnesium material without special processing is lower than this and the general elongation is about 10%.

In the case of the 30-minute treated material subjected to more annealing treatment, the strength was somewhat reduced compared to the two-minute treated material, but the maximum elongation reached 40%.

In addition, the recrystallization treatment after the double-speed rolling can obtain fine grains of equiaxed crystals, and the above-mentioned ductility can be improved and superplasticity capable of plastic processing at a high processing speed can be expected. It was confirmed that the lower than the normal temperature, or the molding speed can be higher than the normal molding speed.

In addition, although not shown in the above table, when the annealing time is excessive, the elongation was rather decreased, and the strength was also decreased, which was determined to be due to the loss of the microcrystalline structure due to the grain growth.

As can be seen from the above, according to the present invention, the material can be subjected to double-speed rolling without heating the material at all, or heating to a low temperature of 100 ° C. or lower, so that magnesium plate can be economically produced. The prepared magnesium alloy is excellent in strength, ductility and superplasticity, which is advantageous for processing into a complicated shaped article.

Claims (6)

Preparing a magnesium plate preheated to room temperature or below 100 ° C .; Performing a continuous rolling on the magnesium sheet; And Annealing the accelerated rolled magnesium sheet; Method for producing a high strength, high ductility magnesium sheet, characterized in that consisting of. The method of claim 1, wherein the rotational speed ratio of the two rolling rolls facing each other during the continuous rolling is between 1.1: 1 and 10: 1. The method according to claim 1, wherein the rolling reduction rate per pass during the continuous rolling is 5 to 30%. The method according to claim 1, wherein when the preheating temperature of the magnesium sheet is 60 to 100 ° C, the reduction rate per pass during the continuous rolling is 50% or less. The method for producing a high strength, high ductility magnesium sheet according to any one of claims 1 to 4, wherein the temperature of the annealing treatment is 100 to 450 ° C. The method of claim 5, wherein the processing time of the annealing treatment is 2 minutes to 4 hours.

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